Human Activities Trigger Explosion of Infectious Diseases

Get used to SARS, West Nile, hantavirus, Ebola, Nipah, Hendra, AIDS and other nasty infectious diseases. Health experts say we're living in a new age of infections.

And we have mostly ourselves to blame.

The nation's top scientists say that environmental, economic, social and scientific changes have helped to trigger an unprecedented explosion of more than 35 new infectious diseases that have burst upon the world in the past 30 years.

The U.S. death rate from infectious disease, which dropped in the first part of the 20th century and then stabilized, is now double what it was in 1980.

SARS is only the latest of these new germs.

"The kinds of things we are doing for SARS, we can anticipate we are going to do again and again," Centers for Disease Control and Prevention Director Dr. Julie Gerberding told the U.S. Senate's Health, Education, Labor and Pensions Committee this week.

Even as most of the world struggles to cope with the threat of severe acute respiratory syndrome, infectious disease specialists are warily eyeing an outbreak of a bird influenza in the Netherlands. Only one person has died so far, but forms of flu that are new to humans, as this one is, can be deadly.

"Things are closing in on all sides," said Dr. W. Paul Glezen, an epidemiologist at the Influenza Research Center at Baylor College of Medicine in Houston. "Today's outlook with regard to microbial threats to health is bleak," concludes a 396-page March report from the prestigious Institute of Medicine, a research arm of the federal government.

"Microbial threats present us with new surprises every year."

Every hour 1,500 people worldwide die of an old or new infectious disease, and more than half of those are children under 5, according to that report.

Many of the infectious diseases that now seem common - food-borne E. coli, waterborne cryptosporidium, airborne Legionnaires' disease, blood-borne hepatitis C and sexually transmitted AIDS - first surfaced in roughly the past 30 years.

"This period from the 1970s is without precedent in the history of the annals of medicine," said Dr. Paul Epstein, associate director of Harvard Medical School's Center for Health and Global Environment.

To figure out why this is happening, the Institute of Medicine convened a panel of top U.S. researchers. They attributed the surge in new diseases to 13 specific changes in the world and the way we live. Those factors are microbial adaptation and change; human susceptibility to infection; climate and weather; changing ecosystems; human demographics and behavior; economic development and land use; international travel and commerce; technology and industry; breakdown of public health measures; poverty and social inequality; war and famine; lack of political will; and bioterrorism.

Of the more than 35 emerging diseases since the 1970s, "a substantial proportion relate to man's manipulation of ecology," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in an interview Thursday.

"It's a confluence of many factors," said Dr. Fred Sparling, a medical and microbiology professor at the University of North Carolina at Chapel Hill and a co-author of the IOM report. "We have more people in mega-cities, which increases the risk for dissemination, more people moving into habitat that is used by animals as vectors for transmission" of diseases, "more international travel and commerce," which disseminates germs once they have arisen. "And more poverty and social inequalities, which clearly add to it."

Most of these diseases live in animals; the epidemic begins when they jump from animal to humans, typically because of close contact. That often follows the clearing of land for housing or industry - in effect putting people closer to the ticks, mosquitoes and rodents that carry new diseases.

Lyme disease soared in the United States after land clearing in the Northeast drew white-tailed deer closer to humans, said Stanford University medicine Professor Dr. Stan Deresinski, editor of Infectious Disease Alert. Lyme-carrying ticks, which lived on those deer, could more easily reach humans and infect them.

New dams cause changes in water flow, often increasing mosquito populations. In 1977 and 1978, for example, standing water behind Egypt's Aswan Dam provided a spectacular ground for the mosquito that carried Rift Valley fever, North Carolina's Sparling said. The disease infected 200,000 people and killed 598.

Dense urban populations, people living longer and the increased number of immune-compromised people also help diseases spread faster, the IOM report said.

When countries hide new outbreaks, as China did with SARS, orlack public health systems capable of detecting outbreaks, diseases spread faster, said Dr. Howard Markel, a professor of infectious diseases at the University of Michigan. Markel, who also teaches the history of medicine, is author of the forthcoming book "When Germs Travel."

Weather changes can be a factor in emerging diseases too, experts said. West Nile and hantavirus struck in the United States after severe droughts, Epstein said. That's because transfers of mosquito-borne diseases are easier when large numbers of animals congregate around small pools of water.

Another human factor: Many new diseases are strains of old diseases that have developed resistance to antibiotics, Sparling said.

Once a disease takes hold these days, it tends to be globalized quickly by travel and trade.

West Nile, for example, is thought to have reached New York from its traditional home in the Middle East on an infected bird carried by a ship or plane. With SARS, doctors have been able to pinpoint tourists, businessmen and doctors who have taken the virus from Hong Kong to Hanoi, Singapore and Toronto. It set a record for speed of continent-to-continent transmission.

This is the first widespread emerging disease that's airborne, "and that's what has everybody afraid," Epstein said. Although SARS isn't as contagious as measles or even the common flu, airborne transmission makes it easier to spread than diseases requiring a mosquito bite such as West Nile.

"It's amazing; it's scary," said Dr. Jim Diaz, professor of public health and preventive medicine at the Louisiana State University School of Medicine. "It just shows you that international travel can move things so fast."

Fortunately, the world's health systems are moving faster too. The CDC's Gerberding noted that within about a month, scientists identified the SARS virus, something never seen before, and developed a crude diagnostic test for the disease.

The essential goal is to keep infections from getting out of control.

"We don't conquer germs," said Michigan's Markel. "We wrestle them to a draw."